The use of fossil fuels for power generation grows increasingly problematic. First, petroleum consumption has increased even as world-wide petroleum reserves have declined. For example, Saudi Arabia's domestic petroleum consumption due to power generation is expected to be 8 million barrels/day by 2028, which means a reduction of the quantities available for export. Second, concerns about air quality may result in stringent regulations such as a carbon tax aimed at reducing carbon emissions.
Given Saudi Arabia's abundant quantities of solar radiation energy, solar power capture coupled with solar storage represents an opportunity to address both issues. Conventional solar storage and capture systems include photovoltaics and solar thermal systems.
Photovoltaics convert solar energy into electrical current due to the photovoltaic effect of certain substances, such as silicon or organic solar materials. Photovoltaics are capital intensive, but excellent for small scale electricity generation, for example, homes, outdoor lights, highway signs. For larger systems, such as those that contribute to the electricity grid, solar thermal systems, or concentrating solar power (CSP) systems, are preferred. Existing CSP systems include, for example, the linear Fresnel reflector system, the trough system, the dish system, and the tower system.
CSP systems convert solar radiation energy into thermal energy using heliostats. Heliostats are mirrors, typically flat, which are mounted such that they move on an axis to track the movement of the sun during daylight hours. Heliostats concentrate the solar radiation (sunlight) onto a receiver, which uses the thermal energy from the solar radiation to heat a working fluid. The working fluid, a heat transfer fluid, such as water (H2O) or molten salt, exits the heliostat/receiver system where it exchanges heat with H2O to generate steam. When H2O is the working fluid, the steam is generated directly from the heated working fluid. The steam runs a steam turbine, which drives a generator to produce electricity.
All CSPs operate under the same basic principles, the differences lie in the shape and layout of the heliostats and the spatial relationship of the heliostats to the receiver. For example, in a linear Fresnel reflector system, the heliostats are long flat tracks of mirrors. The receiver is a tube fixed in space above the mirrors. A trough system uses parabolic mirrors and a tube positioned along the focal line of the reflectors, requiring a large number of reflectors. Dish system CSPs also use parabolic shaped reflectors; a large parabolic dish covered in mirrors directs sunlight to a receiver mounted on the dish along the focal line of the mirrors. A dish system CSP produces relatively little electricity compared to other CSP systems. Tower system CSPs employ large numbers of heliostats typically arrayed in lines. The receiver sits on the top of a tall tower and the heliostats focus the solar energy onto the receiver. A tower CSP is capable of producing up to 200 megawatts of electricity.
In addition to the ability to generate large amounts of electricity, another advantage of solar thermal systems over photovoltaics is the ability to store thermal energy in the working fluid. The working fluids may be stored in tanks until the thermal energy is needed for electricity generation. Thus, allowing generation even when there is no direct sunlight, such as at night or in stormy weather. Even still, the storage of a working fluid is not a long term solution, due to the size of the tanks needed for storage and eventual heat loss. Thus, the conversion of solar thermal energy to fuel is an attractive alternative.
The emission of CO2 into the atmosphere is increasingly under attack. Carbon capture technologies are being explored as a way to remove and store the CO2 from waste gas. Carbon capture technologies are broadly categorized as to whether the capture technology is post-combustion, pre-combustion, or oxyfuel combustion. Post-combustion technologies typically include solvent capture systems, which use a solvent to absorb CO2 from a waste gas stream and then use heat to remove the absorbed CO2 from the solvent stream. The resulting stream is a nearly pure stream of CO2. Post-combustion technologies are commonly used with fossil fuel burning power plants. Other post-combustions technologies include, for example, calcium looping cycle or chemical looping combustion.
Current storage (or sequestration) schemes most commonly include geological sequestration, in which the carbon is stored in underground formations. Depleted oilfields, unmineable coal deposits, and saline formations provide naturally occurring formations appropriate for the storage of CO2. These formations, however, suffer from setbacks including, for example, their locations, the costs to inject the CO2 into the ground, and the concerns about leakage out of the formation at some later point.
An alternative to sequestration of CO2 is to convert the CO2 to other useful components. One way to achieve conversion is using a fuel cell to convert the CO2 with the added benefit of generating electricity. Fuel cells contain three sections: an anode, a cathode, and an electrolyte. Redox reactions occur at the anode and the cathode. In many cases, the overall effect is to convert H2O to hydrogen (H2) and oxygen (O2).
Fuel cells are categorized by their electrolyte. One category of fuel cells uses a solid oxide electrolyte. Solid oxide fuel cells reduce oxygen on the cathode side, a current is applied to the cathode so that it is negatively charged and conductive. The oxygen ions diffuse through the cathode, the solid oxide electrolyte, and the anode so that oxidation reactions occur on the anode side. The oxidation reactions generate electrons which can be carried through the anode to generate an electricity supply. The anode, cathode, and solid oxide electrolyte of solid oxide fuel cells are composed of ceramic materials and operated at temperatures above 500° C. to ensure the proper functioning of the ceramic materials. The ceramic materials can be porous. Porosity is not required for the passage of oxygen ions from the electrode to the electrolyte. The porosity of the anode impacts the electrolyte/electrode/gas interface area (three phase boundaries), and thus impacts oxygen ion formation rate. The porosity also enhances the diffusivity of molecular oxygen from the gas phase to the three phase boundaries. Solid oxide fuel cells have been shown to have high efficiencies.
A solid oxide fuel cell run in a “regenerative” mode is often called a solid oxide electrolysis cells. Solid oxide electrolysis cells electrolyze components by a reduction process on the cathode side, thus capturing oxygen ions, which diffuse through the cathode, the solid oxide electrolyte, and the anode to form oxygen molecules on the anode side of the cell. The electrolysis of H2O is endothermic, thus the high operating temperatures of a solid oxide electrolysis cell make the electrolysis reaction thermodynamically favored. In addition, the high temperature increases the kinetics of the reaction. High temperature electrolysis has the advantage of high conversion efficiency, above 90% conversion of CO2 is expected according to some estimates.